1. Technical Field
Aspects of the present invention relate to a power semiconductor module or device applied to a multi-level power conversion device of three levels or more, and to a power conversion device to which the module is applied.
2. Related Art
FIG. 16 shows an example of a circuit of a three level inverter, which is a power conversion circuit that converts from a direct current to an alternating current. In a direct current power source in which C1 and C2 are connected in series (a large capacity capacitor may be used instead), a positive side potential is Cp, a negative side potential is Cn, and intermediate point potentials are Cm (Cm1 and Cm2). Generally, when the direct current power source is configured from an alternating current power source system, it is possible to configure by applying a rectifier, a large capacity electrolytic capacitor, or the like.
An upper arm is configured of an IGBT 3 and a diode 4 connected to the positive side potential Cp, and a lower arm of an IGBT 5 and a diode 6 connected to the negative side potential Cn. The upper arm and lower arm are connected in series, configuring a one phase arm 24. Phase arms 25 and 26 also having the same configuration, a three phase circuit is configured of the three phase arms. Also, reference numerals 7, 8, 9, and 10 are elements configuring a bidirectional switch connected between the direct current power supply intermediate point potential Cm and an alternating current output terminal 11, wherein 7 and 8 are IGBTs, and 9 and 10 are diodes. The bidirectional switch shown in FIG. 16 is of a configuration wherein IGBTs to which a diode is reverse parallel connected are connected in reverse series, and is applied to each phase. In the drawing, the IGBT 7 and IGBT 8 are connected in reverse series with a common emitter, but the switch can also be realized with a common collector configuration or, as shown in FIG. 18B, with a configuration wherein IGBTs 12 and 13 having reverse blocking voltage are reverse parallel connected.
Lo is a filter reactor, and 2 is a load of the system. By adopting this circuit configuration, it is possible to output the direct current power source positive side potential Cp, negative side potential Cn, and intermediate point potential Cm, to the output terminal 11. That is, the circuit is a three level inverter circuit that outputs three levels of voltage waveform. FIG. 17 shows an example of an output voltage (Vout) waveform. A characteristic being that there are less low order harmonic components (close to a sinusoidal waveform) than with a two level inverter, it is possible to miniaturize the output filter reactor Lo.
Also, FIG. 19 shows a double converter type of power conversion system configured of a PWM converter (CONV) that converts alternating current to direct current and a PWM inverter (INV) that converts direct current to alternating current. A configuration is such that, with a three phase alternating current power source 1 as an input, a stable alternating current voltage is generated by an input filter reactor Li, the three phase three level PWM converter CONV, large capacity capacitors C1 and C2 connected in series, the three phase three level PWM inverter INV, and an output filter Lo, and alternating current power is supplied to a load 2.
When configuring the three level converter circuit (converter or inverter) shown in FIG. 16 with current commercially available IGBT modules, the phase arms 24, 25, and 26 are configured with a 2 in 1 type of IGBT module, and bidirectional switch IGBT modules 27 to 32 with a 1 in 1 type of IGBT module. As an example of a 2 in 1 type of module, an appearance thereof is shown in FIGS. 20A and 20B, and an internal circuit configuration thereof in FIG. 21. FIG. 20A is a type wherein output terminals are installed in the upper portion of the module, and FIG. 20B is a type wherein output terminals are installed in end edge sides of the module. As the output terminals, there is a terminal P (33 and 34) connected to the direct current power source positive side potential Cp, a terminal N (35 and 36) connected to the negative side potential Cn, and a terminal U (37, 38a and 38b) connected to the load output and bidirectional switch elements, and the terminals are generally configured in the order shown in the drawings. Herein, as the terminal U has a larger current capacity than that of the terminal P and terminal N, it has a two terminal structure in FIG. 20B. Also, an example of a 1 in 1 type of module and an internal circuit configuration thereof are shown in FIGS. 22 and 23. The output terminals are configured of a collector terminal 39 and an emitter terminal 40.
FIG. 24 shows an example of a structural diagram (a top view) when configuring one phase of the circuit shown in FIG. 16 using these modules. The drawing shows an example configured of the type of module MJ2-1 (a 2 in 1 type) of FIG. 20A, the module MJ1-1 (a 1 in 1 type) shown in FIG. 22, and electrolytic capacitors C1 and C2 forming a direct current power source, each of which is wired by a copper bar (conductors A to E). The wiring form is inevitably complicated due to the terminal positions of the modules, and the wires between the modules (MJ2-1 and MJ1-1) and the electrolytic capacitors C1 and C2 are long. The tendency is the same when applying the type of module in FIG. 20B too.
A main circuit configuration of a three level inverter is shown in Japanese Patent Publication No. JP-A-2008-193779, and an external form and configuration diagram of a heretofore known module are shown in Non-patent Document 1: “FUJI Power Semiconductors IGBT Modules” by Fuji Electric Device Technology Co., Ltd., March, 2010, PMJ01e.
When the wires between the semiconductor modules and the direct current power source are long, as heretofore described, a problem occurs in that wire inductance increases, and a surge voltage at a time of a switching action becomes excessive.
FIG. 25 shows an equivalent circuit described focusing on the wire inductance of the one phase circuit of FIG. 16. Each inductor (L1 to L5) is mainly formed by the wires between the modules and between the modules and the direct current power source (the capacitors C1 and C2). As each wire is normally of the order of a few centimeters to around dozen centimeters, each inductance value is of the order of 10 nH to a few tens of nanohenrys.
In FIG. 25, with an IGBT 3 in an on condition, a current I with the path shown by the dotted line flows along a path from the direct current power source C1, through the inductor L1, the IGBT 3, and a reactor Lo, to the direct current power source C1. From this condition, on the IGBT 3 being turned off, an IGBT 7 (turned on in advance) and a diode 10 have continuity, and the current of the reactor Lo is transferred to a path 41 of the inductor L2, through the IGBT 7, the inductor L3, a diode 10, and the inductor L4, to the reactor Lo. At this time, a voltage is transiently generated in the directions of the arrows in the drawing in the inductors L1, L2, L3, and L4, in accordance with an IGBT current change rate (di/dt).
As a result of this, a maximum of the voltage shown in Equation 1 is applied between the collector and emitter of the IGBT 3. FIG. 26 shows waveforms of a collector current (ic) and a voltage between the collector and emitter (VCE) when the IGBT 3 is turned off.VCE(peak)=Edp+(L1+L2+L3+L4)·di/dt  Equation 1Surge voltage ΔV=(L1+L2+L3+L4)·di/dt  Equation 2
Edp: direct current power source 1 direct current voltage
di/dt: IGBT current change rate when IGBT is turned off
L1, L2, L3, and L4: wire inductance value
As one example, in the case of an IGBT in the few hundred ampere class, as the current change rate di/dt thereof is a maximum of around 5,000 A/μs, when L1+L2+L3+L4=100 nH, the surge voltage (L1+L2+L3+L4)·di/dt according to Equation 1 is 500V.
Consequently, due to the existence of L1, L2, L3, and L4, the value of the peak voltage applied to the IGBT when the IGBT is turned off is a high voltage value wherein the surge voltage in Equation 2 is added to the direct current voltage (Edp). As a result of this, elements with a high voltage rating are necessary for the IGBT chip and the diode chip connected in parallel thereto. Normally, a chip with a high amount of voltage resistance is such that the chip area increases roughly in proportion to the voltage rating, meaning that there is a problem in that the power semiconductor module increases in size, and the cost increases. Also, there is a problem in that a conversion device in which the power semiconductor module is used is also large, and is high-priced.